1. Field of the Invention
The present invention relates to a liquid absorption agent for water or organic solvents comprising, as a main component, a crosslinked N-vinyl-carboxylic acid amide resin comprising the backbone (or main) chain of a homopolymer or copolymer containing at least 50 mole % of an N-vinylcarboxylic acid amide component crosslinked with a crosslinking agent. More specifically, the present invention relates to a liquid absorption agent having the following characteristics i.e., it is chemically stable, has an excellent absorption for water and organic solvents such as alcohol, particularly a good absorption for a liquid in which metal ions and organic ions co-exist in the system, and its absorbability is little influenced by the co-existing ions. Also, as a result of the absorption of a liquid, it is itself swelled or gelled to non-fluidize and solidify the co-existing liquid system, and exhibits a slow releasability and adhesion, thus having wide applications in various fields by making use of the excellent characteristics and functions of the crosslinked N-vinylcarboxylic acid amide resin.
2. Description of the Related Art
Liquid absorptive resins are used widely in such fields as medicine, sanitation, food industry, agriculture, horticulture, and civil engineering, and in all cases, there is a demand for a high swelling ratio and gel strength. Examples of water absorptive resins known in the art include hydrolyzates of starch-acrylonitril graft copolymers, neutralized products of starch-acrylic acid graft copolymers, saponified products of vinyl acetate-acrylate copolymers, hydrolyzates of acrylonitrile or acrylamide type crosslinked copolymers, and polyacrylic acid salt type crosslinked polymers or the like. These absorptive resins, however, are all polymeric electrolyte type crosslinked polymers, and therefore, although they exhibit an excellent swelling ability for water not containing any electrolyte, they exhibit a remarkably low swelling ability for an aqueous liquid containing a large amount of electrolytes such as blood, urine, aqueous fertilizer, and cement slurry. This is considered to be because a dissociation of the polymeric electrolyte, which is the backbone chain of the crosslinked polymer, is suppressed, and thus the expansion of the chain is reduced. Further, when polyvalent metal ions exist, a further ion crosslinking occurs by the carboxylic acid moiety in,the backbone chain carboxylic acid to form a crosslinked polymer with a substantially higher crosslinking density than is necessary, which also causes a lowering of the swelling ratio. To avoid such drawbacks, for example, Japanese Unexamined Patent Publication (Kokai) No. 61-97312 discloses a method of preparing a water absorptive resin containing carboxyl groups in the chain structural units, in which an acrylic acid type compound is graft polymerized onto hydroxyethylcellulose, followed by hydrolysis. This method obtains an ion-resistant water absorptive resin by introducing nonionic polymers into a polymeric electrolyte backbone chain, but is not always satisfactory with respect to chemical stability of the backbone chain or the simplicity of the preparation method. Japanese Unexamined Patent Publication (Kokai) No. 60-55011 discloses a method of preparing an absorptive resin having an improved absorption ability for an aqueous electrolyte such as an aqueous sodium chloride, by a copolymerization of three kinds of compounds of (meth)acrylamide type compound, (meth)acrylic acid type compound and (meth)acrylic compound having a sulfonic group, in the presence of a divinyl type compound. Japanese Unexamined Patent Publication (Kokai) No. 58-5305 discloses a water-swellable crosslinked polymer containing an N-vinyl compound, but since each polymer is intended to avoid an inhibition of a spreading of the backbone chain in an electrolyte solution by an introduction of relatively strong ionic dissociation groups, (meth)acrylamide or an N-vinyl compound is not substantially the main component, and thus a water absorptive resin with a high ion resistance cannot be always obtained. Further, as described above, because the backbone chain is a polymeric electrolyte, a large amount of electrolytes must be contained in the swelled gel, and accordingly, the gel does not always exhibit a sufficient effect as a water supplementing material for agricultural and horticultural use, and there is a demand for a material exhibiting a high function in this field. Similarly, these water supplementing materials also must have an excellent light resistance, due to the uses thereof, but a satisfactory solution has not been obtained also in this respect.
The water absorptive resins, as is obvious, form gelled products by an absorption of water, but exhibit no swellability with organic solvents such as alcohol, and the use thereof has been limited to, for example, water absorption and water retention.